The performance and characteristics of an electronic or a photonic device usually depend on the temperature at which the device is operated. The function of certain devices is sometimes based on this dependence. A well-known example is the red-shift of the emission wavelength of a distributed feedback (DFB) semiconductor laser with increasing temperature. This dependence can be used to maintain the emission wavelength to a specific value, as may be desirable for instance in a communication system, or to tune the emission wavelength over a spectral range overlapping with an absorption feature of an analyte, as in most laser-based gas sensors.
Currently, DFB quantum cascade lasers (QCLs) (see, e.g., C. Gmachl et al. IEEE J. of Quantum Electron. 38, 569 (2002), which is incorporated herein by reference in its entirety) are the most widely used single mode semiconductor laser sources in the mid-infrared portion of the electromagnetic spectrum. The optical cavity of these devices incorporates a Bragg grating acting as a wavelength filter that results in single mode emission. Distributed Bragg reflector (DBR) lasers and sampled grating DBR (SG-DBR) lasers are other well-known examples of devices having a single mode emission. Unlike DFB lasers however, these devices are composed of several independent sections, including typically a gain section, one or more sections with a Bragg grating and one phase shift section as discussed in L. Coldren and S. Corzine, Diode Lasers and Photonic Integrated Circuits, New York Wiley, 1995. Wavelength tuning in single mode QCLs is achieved by changing the temperature of the optical waveguide as a whole or in selected parts of the laser cavity, especially where gratings are present. The temperature change translates in a change of the mode effective refractive index, which is directly related to the wavelength selected by a Bragg grating.
Changing the temperature of a QCL waveguide results in wavelength tuning. This is usually achieved by two different mechanisms, also commonly used to tune the emission wavelength of other types of lasers such as diode lasers:
(1) Direct Current Tuning
Changing the current flowing through a QCL changes typically its core temperature by a large amount, because the QCL gain medium (1) requires high current (0.5 to 4 Amp.) and voltage (8 to 18 Volts), i.e., high input electrical power, (2) is highly inefficient, e.g. 80% to 90% of the electrical power provided to the laser is not transformed into photons but instead is dissipated in heat and (3) has very poor thermal conductivity on the order of 2 W/(m K). Fast tuning rates reaching hundreds of kHz and more is possible with this mechanism, because the temperature changes take place only in a very small volume of the device. However, the tuning achievable is rather limited, and therefore the tuning range achievable by this mechanism is bound to only 3 cm−1 to 6 cm−1. This limit originates from the fact that QCLs operate (i.e. emit light efficiently) only up to certain operating current, after which the laser intensity starts to decrease sharply (roll over). The maximum operating current, and therefore the maximum electrical power dissipated in the waveguide core translates into a maximum temperature change of typically less than 50K. For more details, see, e.g., C. Gmachl et al. Optics Lett. 25 230 (2000).
(2) Heatsink Temperature Tuning
Wavelength tuning can be achieved by changing the temperature of the DFB QCL chip as a whole, through a temperature variation of the mount and/or heatsink to which the laser chip is attached. As discussed in A. Wittman, et al., IEEE Photon. Tech. Lett., 21 814 (2009), which is incorporated herein by reference in its entirety, tuning range reaching ˜15 cm−1 in the long-wave infrared or LWIR (i.e. λ˜8-12 μm) can be achieved by varying the heatsink temperature from 245K to 425K. This corresponds to a tuning coefficient in the range of 0.075 cm−1/K. Slightly broader tuning (˜20 cm−1) was achieved at shorter wavelength (i.e. λ<5 μm), as discussed in J. S. Yu et al., Appl. Phys. Lett. 87, 41104 (2005), which is incorporated herein by reference in its entirety. This is a significantly broader tuning range compared to the value achievable by current tuning alone. Tuning the temperature of the heatsink is not limited by the maximum current that the laser can sustain, but rather by the much larger temperature range over which the laser can reach threshold (200K or more). This mechanism is very convenient but the tuning speed is typically extremely slow, on the order of seconds up to a few minutes. This is due to the long time necessary to heat up and cool down the large mass of the heatsink and the other components to which the laser is directly or indirectly connected. In the case of photonic chips that include an ensemble of temperature-sensitive elements (for example a DFB QCL array as described in U.S. Pat. No. 7,826,509 B2 to Belkin et al., which is incorporated herein by reference in its entirety), this tuning method forces the operating temperature of the different elements to change by a similar value, which may not be desirable for some applications.
Another tuning method involves integrating resistive thin film heaters directly on top of passive sections of DBR lasers. This approach is most commonly applied to single mode diode lasers and has been demonstrated in the early 1990's as discussed for example in S. Sakano et al., IEEE Photon. Tech. Lett. 4, 321 (1992), 1T. Kameda et al., IEEE Photon. Tech. Lett. 5, 608 (1993), and F. A. Kish et al, US patent 2005/0018721A1, each of which is incorporated herein by reference in its entirety. Resistive thin film heaters include typically a metal such as Au, Pt, Pt/Ti, NiCr, TaN and an insulator deposited next to or directly on top of the electric contacts used to control the device below. The insulator prevents an electric short between the heater and other parts of the laser, in particular electrodes. Joule heating takes place as current is injected in the thin, resistive metal layer, which results in a temperature increase and hence a refractive index change in the DBR laser. This approach however does not easily allow for the large temperature changes (100K and higher) desirable for QCLs because of the limited electrical power (typically less than a few Watts) that a thin film can sustain before degrading, causing reliability problems. Finally, the fact that the heater is fabricated directly on top of the device may also prevent epitaxial-side down mounting.